Process and apparatus for automatic drilling

ABSTRACT

The process comprises progressively increasing the weight on the drill bit, determining the greatest value of the penetration rate of the drill bit during this period, progressively decreasing the weight on the drill bit when the penetration rate has reached a determined value and again progressively increasing the weight on the drill bit when the tension on the drill pipe has reached a fixed value. The passage from a period during which this weight decreased and vice-versa is achieved by varying the linear speed of the drill pipe at the ground surface, whereby the tension on the drill pipe is varied.

0 llntte States .1 atet 1191 1111 3,872,932 Gosselin Mar. 25, 1975 [5 PROCESS AND APPARATUS FOR 3,648,783 3/1972 Lackey .1 173/4 AUTOMATHC DRILLING 3,658,138 4/1972 Gosselin 173/6 3,659,655 5/1972 Gyongyosi Inventor: Jean-Charles Gosselin, Versailles, 3,800,277 3/1974 Patton et a1. .1 175/26 France [73] Assignee: Institut Francais du Petrole des Primary E-"ami'ler james PelpPink Carbumms et Lubrifiams Attorney, Agent, or Firm--Cra1g & Antonelli Rueil-Malmaison, France [22] Filed: Oct. 23, 1973 [57] ABSTRACT The process comprises progressively Increasing the 1 pp ,261 weight on the drill bit, determining the greatest value of the penetration rate of the drill bit during this pe- 52 us. or 173/1, 173/6, 175/26, riod progressively decreasing the Weight on theidri" 175/103 bit when the penetration rate has reached a deter- 1511 1m. (:1 E21b 3/12 mined Value and again progressively increasing h [58] Field of Search 173/1, 4 6; 175/271 40, weight on the drill bit when the tension on the drill 175/57 pipe has reached a fixed value. The passage from 21 period during which this weight decreased and vice-versa [56] References Cited is achieved by varying the linear speed of the drill pipe UNITED STATES PATENTS at the ground surface, whereby the tension on the drill V pipe is varied. 3,039,543 6/1962 Loocke 173/4 3,550,697 12/1970 HObhOUSB 175/26 12 Claims, 6 Drawing Figures PATENTEDHARZ 519. 5

SHEETIUFA FIG] PROCESS AND APPARATUS FOR AUTOMATIC DRILLING The present invention relates to a process and an apparatus for automatically optimizing the penetration rate of a drilling tool driven by a motor whose torque decreases with an increasing rotation speed.

Such relationship between the torque and the penetration rate characterizes, in particular, the bottom or down-hole motors, such as the drilling turbines used in the turbodrilling process, or the series or compound electric drilling motors used in the electrodrilling pro cess.

As a result of the above-defined relationship, it is observed that, when starting from a low value of the thrust load or weight applied on the drill bit, the penetration rate begins to increase with a progressively increasing weight on the drill bit and then decreases with a further increasing weight on the drill bit, until stalling of the drill bit.

It has already been proposed, in drilling operations using a motor whose torque decreases with an increasing rotation speed, to progressively vary the load or weight applied on the drill bit, about its optimum value, in alternating series of phases furing which is repectively increased and decreased the tension of the drill pipe from which the drill bit is suspended, to measure the slope of the tangent to the curve which represents the penetration rate of the drill bit into the ground as a function of the weight exerted on the drill bit and to change over from a tension increasing phase to a tension decreasing phase and vice-versa, when the value of said slope reaches respectively predetermined upper and lower values.

The problem arising when carring out such a process and making use of the corresponding apparatus are due, in particular, to the great difficulty encountered in determining the slope of the curve at the operating point.

The main object of the present invention is accordingly to provide a more easily operable process and apparatus.

The advantages of this invention will become apparent from the following description of an embodiment of the invention illustrated by the accompanying drawings wherein:

FIG. 1 shows, in the case of drilling with a bottom motor, the law of variation of the rotation speed of the drill bit, in response to the weight exerted thereon,

FIG. 2 diagrammatically illustrates the variations of the penetration rate and of the pressure of the drilling fluid versus time,

FIG. 3 shows an apparatus according to the invention for use when the drill pipe is a flexible pipe,

FIG. 3A shows the adaptation of this apparatus to a drilling process using rigid drill pipe elements,

FIGS. 4 and 5 illustrate the operation of the device.

The curves 1 and 2 of FIG. 1 show for two different ground layers respectively, the penetration rate V of the drill bit in. response to the axial load or weight W exerted on this drill bit.

On these curves the points M and M indicate the respective operating points when the drill bit penetrates the corresponding ground layers and the abscissa W; represents the weight (or load) exerted on the drill bit.

It is apparent that with an increasing weight Wf on the drill bit, starting from the operating point M or M the corresponding operating point is displaced towards the right of FIG. 1 on either of the two curves and the penetration rate V of the drill bit increases and reaches a maximum value at point S, (or S The corresponding curve portion followed by the operating point corresponds to stable operating conditions.

If the operating point moves beyond the vertex of the operating curve the operating conditions become unstable and the penetration rate decreases with a further increasing weight W on the drill bit, until stalling of the downhole motor.

FIG. 2 represents, as a function of time, the variations in the value of the penetration rate V, and in the pressure of the drilling mud at the ground surface.

The penetration rate V of the drill bit undergoes fluctuations about an average value, these variations in curve 3 being plotted versus time.

The frequency of these fluctuations depends, in particular, on the pulsing rate of the drilling mud circulation pumps and cannot be filtered. without introducing a time constant which is incompatible with the response time required for an automatic apparatus for optimizing the penetration rate of the drill bit, which device must prevent any liability of stalling of the bit.

I will indicate hereinafter how this inconvenience can be avoided by considering the average value of the penetration rate of the drill bit over a time interval equal to the pulsing period of the mud circulation pumps, for example.

The instants t,, and 1,, which are the limits of such intervals may be selected as being the instants at which the value ofthe alternating component of the pressure P,- of the drilling fluid, as measured at the ground surface, passes through the value zero (FIG. 2), the value of this pressure P also oscillating about an average value shown by the straight line 4, at the same frequency as the penetration rate, but with a time lag (b corresponding to the lag between the respective values of the drilling mud pressure at the ground surface and at the hole bottom.

The value of the average penetration rate at the hole bottom between instants t,, and t,, is given by the formula:

wherein is the average value of the linear speed of the drill pipe, as measured at the surface between the two considered instants, a the elasticity coefficient of drill pipe, T,, and T,, the respective tensile stresses or tensions to which the drill pipe is subjected at instant I,, and instant 1,, respectively.

FIGS. 3 and 3A illustrate two embodiments of apparatus for carrying out the process according to the in vention:

FIG. 3 illustrates the case of use ofa flexible drill pipe unwound from a storing reel and FIG. 3A the case of use of a drill string formed of rigid pipe elements.

In these figures, reference 5 designates the drill bit suspended from a drill pipe 7 and driven by a downhole motor 6, which is, for example, a drilling turbine fed with hydraulic power from the ground surface, reference 8 indicating the bore hole.

The drill pipe 7 is lowered in the bore by means of a handling device 9 which may comprise at least one endless caterpillar chain carrying jaws or clamping shoes in the case of FIG. 3, and a winch 33 with a cable 34 wound thereon and supporting a pulley block in the case of use of a drill string 7 formed by rigid drill pipe elements (FIG. 3A).

This handling device 9 is driven by a motor 10, optionally through an irreversible coupling device 11 which can at will be engaged or disengaged by means of electric control signals transmitted through control cable 12 (FIG. 3).

A real time digital computer 13 receives a signal, such as an electric signal representing the measuring value of the tensile stress T applied to the drill string 7. This signal is transmitted through line 14 and can'be supplied, in the case of the embodiment of FIG. 3, by a device 15 comprising at least one strain gauge at the lower part of the drill string 7 (FIG. 3) or at a location 35, on the dead end ofa pulley block 31 at the surface, in the embodiment illustrated by FIG. 3A.

A device 16 is used for measuring the length L of the drill pipe suspended from the ground surface by means of the handling device 9.

This device comprises, for example, a roller in contact with the drill string 7, driven in rotation by the linear displacement of the later, this roller in turn driving in rotation the emitter 16 of a synchromechanism known in the art as a Selsyn, whose receiver 17, to which it is electrically connected through a cable 18, actuates a revolution counter device, providing, in the form of a series of electric pulses, a digital measurement of length L, which is supplied to the computer 13 through cable 19.

The feed rate V,, of the drill string, at the ground surface, can be measured by devices 20 and 21 which are respectively similar to devices 16 and 17, the digital value of said feed rate being supplied to the digital computer 13 through cable 22.

For this measurement of V it is preferred to use devices separate from those used for measuring the length L, so as to determine with a sufficient accuracy the linear speed V,, of the drill string.

The value of the elasticity coefficient a of the drill string 7 is set up at 23 in the computer 13 which also receives, through conductor 24, a signal representing the digital value of the alternating component of the pressure P, of the drilling mud at the gound surface, this pressure being measured by any suitable pressure sensor (not shown in the figure) placed in the drilling mud circuit and producing a measuring signal which is filtered by a device 25 so as to keep only its alternating component which is supplied to the computer 13.

The digital value of a set up at 23 will preferably be a value measured in situ. For this purpose, the average value ofthe coefficient a corresponding to the reached depth may be periodically determined in manner stated' below.

This measurement can be performed by raising the drill bit over a few meters and then lowering it again so that it rests on the well bottom without drilling.

By observing during this lowering of the drill bit the variation in the tensile stress T on the drill pipe, in the linear speed V of this drill pipe at the ground surface, and in the penetration rate V it can be seen that the tensile stress T progressively decreases from instant I,

when the bit reaches the bottom of the bore hole, in

proportion to the elastic shortening of the drill string. This shortening is discontinued at the instant t when the feed rate V, of the ground surface becomes zero.

The relationship:

a L n |I'-I]/( H il-l) will be too low for the right-hand side of the equation (1), the value of thus being the greater in that right-hand side and resulting in a positive value of On the contrary, if the value selected for a is too 'high, the absolute value of the negative term a L( u n-l)/( n ll-I) will be greater than that of and will accordingly result in a negative value of as determined by the computer 13. The exact value of a, measured in situ for the drilled depth, may be determined by the computer 13, by following relationship:

in which I and t e, indicate two instants within the time interval between t, and t L is the drill pipe length supported from the surface, and T and T,- are the respective tensile stresses applied to the drill pipe at instants I;,- and t,,-.

The computer 13 determines, from signals representing the values of V,,, a, L and T, the different successive values of (Va) which are transmitted through a conductor 28 to an apparatus 27 capable of processing such date, in the manner indicated in the following description of the operation of the apparatus.

simultaneously, the value of (V is transmitted to a device 26 through a conducor 30. This device 30 delivers a signal which is a function of the value of V and is supplied to one of the input terminals of the comparator 36 which is a part of the circuit controlling the rotation speed of the motor 10, which circuit will not be described in detail.

The devices 27 and 26, which have been illustrated separately in order to facilitate a better understanding of the operation, can be combined with the computer 13.

The operation of the apparatus will now be de scribed, with reference to FIG. 4.

At the instant t the computer determines the value of the penetration rate Va which corresponds to the operating point M in FIG. 4.

The value of the weight on the c lrill bit is then W the tension in the drill string is T (AT,, Ail =0) and Vd is the value of the linear speed of the drill pipe at the ground surface.

The value Va, is set up in the device 27 and optionally also the corresponding tension T,,.

The conductor 30 supplies to circuit 26 a signal representing the value Va this circuit delivering a signal which is, for example, proportional to the value 2Va which is supplied to one input terminal of the comparator 36, said comparator 36 receiving on another input terminal signal representing the revolution speed of the motor. The comparator 36 delivers a signal which is a function of the difference of the received signals and actuates the control circuit which maintains the speed of the motor 10, at any time, at

This value is transmitted as hereinabove indicated to the means for controlling the linear speed of the drill pipe. This value is also supplied to the device 27 which compares it to the previously recorded value Va for example by, determining the difference (Va,) (Va When this difference is positive the circuit 27 substitutes in the memory the value Va for the value Va and optionally substitutes the value T for the value T This procedure is continued and at the instant the same operation is effected At the instant I, t, m At, the operating point M,,, is substantially at the vertex of the curve. The device 27 thus records a value Va, which is substantially equal to the maximum penetration rate together with the corresponding tension of the drill string.

At the instant r t,,, At t,, (112 +1) At the operating point reaches the position M i.e. the portion of the curve which corresponds to a decrease of the penetration rate as the weight on the tool increases. The corresponding value V,, is accordingly not recorded. since the difference V,,.,,, V is negatwo.

The weight on the drill bit is still increased until the instant r when the operating point M, has reached a position defined by the relationship:

wherein Y represents a set-up value, which is for example supplied to the computer 13 through the conductor 29. At this instant, the computer 13 delivers a control pulse which disengages the device: 11 and, since the device 9 is no longer actuated, the advance of the drill pipe is stopped, thereby resulting in an increase of the tension on the drill pipe and consequently a decrease in the weight on the drill bit.

The operating point is then displaced towards the right of the figure.

The weight on the drill bit decreases and the tension on the drill pipe increases until, at the instant I, the tension on the drill pipe being equal to T, the following relationship is obtained:

AT being a positive set-up value transmitted to the computer 13 through the conductor 37. At the instant .t the computer 13 delivers a pulse which controls the engagement of the device 11, which actuates again the advance ofthe drill pipe at the ground surface and consequently results in a decrease of the tension on the drill pipe. The values V,, and T are recorded in the device 27 and the above-described procedure is repeated, i.e. a new value for v,, is determined at the instant Ar etc.

As is apparent from the foregoing, the computer 13 provides for a determination of the penetration rate (V,,) only during the period when the weight on the drill bit increases (which corresponds to a decrease of the drill string tension).

The above-described process is unaffected by the fluctuations of the valueof V, caused by parasitic phe nomena. Obviously, the value AT will be so selected that the operating point M follows that part of the curve which corresponds to variations in the same direction of the speed V and of the weight W.

According to an alternative embodiment, the values of the tension corresponding to the values of the penetration rate V,, recorded in the device 27 are simultaneously recorded and the computer 13 delivers a pulse which controls the engagement ofthe device 11 for the value of T defined by the relationship:

A'Tbeing the set-up value transmitted through conductor 32 (FIG. 3).

It will then be possible instead of directly setting up reference values AT (or AT) and Y, to adjust the computer 13 so that it elaborates itself the values AT (or AT) and Y as a function of T1 (or m) and of V or of Thus, for example, AT or AT may be a fixed percentage of T, (or of T,,,) and in the same way Y may represent a fraction of V The rate of variation of the weight exerted on the drill bit depends upon the difference between the value of the linear speed (V of the drill pipe at the ground surface and the value of the penetration rate (V,,).

In order to achieve an efficient scanning of the curve V =f(W) by the computer 13, this scanning must be performed over a sufficient time interval; in other words, the variations of the weight on the drill bit must not be too rapid.

To this end the device 11 is omitted and there is used a device 26 which delivers a signal representing the value of V which should be obtained and is a function of V as shown in FIG. 5.

In this figure, curve A represents, by way of example, the function V V curve B represents a function V =F(V such that V is always greater than V and curve C represents a function V, G( V,,), such that V, has a value always smaller than V During the scanning period, when the weight on the drill bit increases, the device 33 delivers a signal represented by curve B [V,, F( V,,)]. When the scanning period of the penetration rate is ended, the computer 13 delivers a pulse transmitted to the device 33 which is triggered and delivers a signal represented by the curve C II G The functions F and G can be selected by those skilled in the art. In the example of FIG. and for values of the penetration rate V greater than a determined limit value:

wherein X and Z are positive constant values expressed by the same units as V It will thus be possible to adjust the period during which the weight on the tool increases (while the tension on the drill pipe decreases, by modifying the value of X.

In order to increase this period it suffices to select smaller values for X.

In order to decrease the period during whicht the weight on the drill tool decreases (while the tension on the drill pipe increases), it suffices to select higher values for Z.

In order to obtain a soft and regular operation for small values of V,, together with scanning periods of sufficient duration, the following values will preferably be selected for F and G.

Thus the linear speed of the drill pipe is easily controlled through conventional control means, and the clutch means 11 of FIGS. 3 and 3A can be omitted.

Obviously the method and apparatus can be used at any time when in the course of drilling operations the relationship between penetration rate of the drill bit and the weight exerted on this drill bit is of the type of the relationship illustrated by FIG. 1.

What I claim is:

l. A process for automatically optimizing the penetration rate of a drill bit suspended from drill pipe, said drill bit being driven in rotation by a motor whose torque decreases with an increasing of its rotation speed, and said drill bit having a penetration rate which, as a function of the weight exerted on the drill bit, reaches a maximum value for an optimum value of the weight, said process comprising measuring the tension on the drill pipe,

periodically determining the penetration rate of the drill bit,

automatically adjusting the linear speed of the drill pipe at the ground surface so as to provide values of the linear speed which are successively greater and lower than the rate of penetration of said drill bit into the ground, the variation in linear speed causing alternatively a decrease and an increase in the tension on the drill pipe, thereby producing an oscillation in the value of the weight on the drill bit about the optimum value of said weight, wherein during each period when the linear speed at the surface is greater than the penetration rate, said penetration rate is periodically determined and the highest value thereof is recorded,

effecting a transition from said period when said linear speed is greater than the penetration rate to a second period during which the linear speed at the surface is lower than the penetration rate when said penetration rate has reached a preselected value, and

effecting a transition to another period wherein the linear speed at the surface has a greater value than the penetration rate when the tension on the drill pipe reaches a preselected limit-value, said tension increasing to said preselected limit-value.

2. A process according to claim 1, wherein a pulsing period is effected by an operating parameter ofthe drilling process, and wherein each determined value of the penetration rate and each measured value of the tension are average values determined over a time interval to said pulsing period.

3. A process according to claim 2, wherein said operating parameter is the drilling fluid pressure.

4. A process according to claim 2, making use of a substantially rigid drill pipe, wherein said operating parameter is an angle of rotation of the drill pipe.

5. A process according to claim 1, wherein said limitvalue of the tension on the drill pipe is determined as a function of the value of the tensile stress in the drill pipe when the penetration rate reaches said recorded highest value.

6. A process according to claim 1, comprising recording the value of the tension on the drill string which corresponds to said highest value of the penetration rate and determining said limit value of the tension as a function of said value of the tension corresponding to said recorded highest value of the penetration rate.

7. A process according to claim 1, wherein the linear speed of the drill pipe is set up at such a value that a difference exists between said linear speed and the penetration rate of the drill bit, this difference being selected as a function of the value of the penetration rate and of the values of parameters which characterize the drilling operation.

8. An apparatus for automatically optimizing the penetration rate of a drill bit located at the lower end of a drill pipe, said drill bit being driven in rotation by a motor whose torque decreases with increasing rotation speed and having a penetration rate which, as a function of the load applied thereon, reaches a maximum value for an optimum value ofthis load, this apparatus comprising means for measuring the tension exerted on the drill pipe, means for evaluating the successive values of the penetration rate and control means for sequentially performing a progressive increase and a progressive decrease of the load on the drill bit, analyzing means connected to said means for evaluating the penetration rate and adapted to determine and record during each period of increasing load the highest value detected by said evaluating means, first actuating means operatively connected to said control means, to said evaluating means and to said analyzing means, said first actuating means elaborating a first limit-value as a function of said highest value recorded by said analyzing means and initiating a decrease in the load exerted on the weight, when the penetration rate reaches, while decreasing, said first limit value and second actuating means operatively connected to said control means, said second actuating means elaborating a second limitvalue and initiating an increase in the load exerted on the drill bit when the tension on the drill pipe has reached, while increasing, said second limit value.

9. An apparatus according to claim 8, wherein said analyzing means comprises comparing means connected to said means for evaluating the penetration rate, and recording means connected to said comparating means which compares each value of the penetration rate to the highest value which has been previously determined during the same period of increase in the load and transfers the greatest of said two values to said recording means where said record value is stored.

10. An apparatus according to claim 8, comprising means for recording the value of the tension corresponding to the highest value of the penetration rate recorded in said analyzing means and wherein said recording means are connected to said second actuating means which elaborate a second limit-value as a function of said recorded value of the tension.

11. An apparatus according to claim 8, wherein said second actuating means are connected to said tension measuring means and elaborate said second limit-value as a function of the tension corresponding to the penetration rate which, while decreasing, has reached said first limit-value.

12. An apparatus according to claim 8, comprising at the ground surface, motor means for advancing the drill string, said motor means being operatively associated with means for automatically regulating the linear speed of the drill pipe, wherein said control means comprise a device connected on the one hand to said elaborating means which delivers a signal representative of the penetration rate and on the other hand to said first and second actuating means, said device delivering a resulting signal which represents the linear speed of the drill pipe at the ground surface and is a function of the penetration rate represented by a signal delivered by said evaluating means, said resulting signal having a value which is lower than the signal representing the penetration rate, when said first actuating means generate a signal transmitted to said device and higher than the signal representing the penetration rate when said second actuating means deliver a signal, and wherein said resulting signal is transmitted to said means for automatically regulating the linear speed of the drill pipe at the ground surface. 

1. A process for automatically optimizing the penetration rate of a drill bit suspended from drill pipe, said drill bit being driven in rotation by a motor whose torque decreases with an increasing of its rotation speed, and said drill bit having a penetration rate which, as a function of the weight exerted on the drill bit, reaches a maximum value for an optimum value of the weight, said process comprising measuring the tension on the drill pipe, periodically determining the penetration rate of the drill bit, automatically adjuSting the linear speed of the drill pipe at the ground surface so as to provide values of the linear speed which are successively greater and lower than the rate of penetration of said drill bit into the ground, the variation in linear speed causing alternatively a decrease and an increase in the tension on the drill pipe, thereby producing an oscillation in the value of the weight on the drill bit about the optimum value of said weight, wherein during each period when the linear speed at the surface is greater than the penetration rate, said penetration rate is periodically determined and the highest value thereof is recorded, effecting a transition from said period when said linear speed is greater than the penetration rate to a second period during which the linear speed at the surface is lower than the penetration rate when said penetration rate has reached a preselected value, and effecting a transition to another period wherein the linear speed at the surface has a greater value than the penetration rate when the tension on the drill pipe reaches a preselected limit-value, said tension increasing to said preselected limitvalue.
 2. A process according to claim 1, wherein a pulsing period is effected by an operating parameter of the drilling process, and wherein each determined value of the penetration rate and each measured value of the tension are average values determined over a time interval to said pulsing period.
 3. A process according to claim 2, wherein said operating parameter is the drilling fluid pressure.
 4. A process according to claim 2, making use of a substantially rigid drill pipe, wherein said operating parameter is an angle of rotation of the drill pipe.
 5. A process according to claim 1, wherein said limit-value of the tension on the drill pipe is determined as a function of the value of the tensile stress in the drill pipe when the penetration rate reaches said recorded highest value.
 6. A process according to claim 1, comprising recording the value of the tension on the drill string which corresponds to said highest value of the penetration rate and determining said limit value of the tension as a function of said value of the tension corresponding to said recorded highest value of the penetration rate.
 7. A process according to claim 1, wherein the linear speed of the drill pipe is set up at such a value that a difference exists between said linear speed and the penetration rate of the drill bit, this difference being selected as a function of the value of the penetration rate and of the values of parameters which characterize the drilling operation.
 8. An apparatus for automatically optimizing the penetration rate of a drill bit located at the lower end of a drill pipe, said drill bit being driven in rotation by a motor whose torque decreases with increasing rotation speed and having a penetration rate which, as a function of the load applied thereon, reaches a maximum value for an optimum value of this load, this apparatus comprising means for measuring the tension exerted on the drill pipe, means for evaluating the successive values of the penetration rate and control means for sequentially performing a progressive increase and a progressive decrease of the load on the drill bit, analyzing means connected to said means for evaluating the penetration rate and adapted to determine and record during each period of increasing load the highest value detected by said evaluating means, first actuating means operatively connected to said control means, to said evaluating means and to said analyzing means, said first actuating means elaborating a first limit-value as a function of said highest value recorded by said analyzing means and initiating a decrease in the load exerted on the weight, when the penetration rate reaches, while decreasing, said first limit value and second actuating means operatively connected to said control means, said second actuating means elaborating a second limit-value and initiating an increase in the load exerted on the drill bit when the tension on the drill pipe has reached, while increasing, said second limit value.
 9. An apparatus according to claim 8, wherein said analyzing means comprises comparing means connected to said means for evaluating the penetration rate, and recording means connected to said comparating means which compares each value of the penetration rate to the highest value which has been previously determined during the same period of increase in the load and transfers the greatest of said two values to said recording means where said record value is stored.
 10. An apparatus according to claim 8, comprising means for recording the value of the tension corresponding to the highest value of the penetration rate recorded in said analyzing means and wherein said recording means are connected to said second actuating means which elaborate a second limit-value as a function of said recorded value of the tension.
 11. An apparatus according to claim 8, wherein said second actuating means are connected to said tension measuring means and elaborate said second limit-value as a function of the tension corresponding to the penetration rate which, while decreasing, has reached said first limit-value.
 12. An apparatus according to claim 8, comprising at the ground surface, motor means for advancing the drill string, said motor means being operatively associated with means for automatically regulating the linear speed of the drill pipe, wherein said control means comprise a device connected on the one hand to said elaborating means which delivers a signal representative of the penetration rate and on the other hand to said first and second actuating means, said device delivering a resulting signal which represents the linear speed of the drill pipe at the ground surface and is a function of the penetration rate represented by a signal delivered by said evaluating means, said resulting signal having a value which is lower than the signal representing the penetration rate, when said first actuating means generate a signal transmitted to said device and higher than the signal representing the penetration rate when said second actuating means deliver a signal, and wherein said resulting signal is transmitted to said means for automatically regulating the linear speed of the drill pipe at the ground surface. 